RF Catheter Tip Forming Systems


 

RF Catheter Tip Forming Theory

RF Catheter Tip Forming Theory Overview

Almost all catheters are exposed to some kind of thermo-forming or bonding. Historically, there have been several methods used to heat the mold for shaping the catheter tip. Follow the link to read more on Catheter Tipping Applications.

Understanding Catheter Tip Forming Process

  1. Catheter is formed by inserting the catheter tubing into a heated mold. As the plastic touches the inner surface of the mold it melts and flows into the mold cavity. After cooling the mold, the plastic solidifies and removed from the mold. The formed tip takes the shape of the mold inner cavity. 
  2. The Mold is heated using an induction coil (See Figure 18). When applying high frequency power through the coil, it creates a magnetic field around the coil which heats the mold.
    INDUCTION COIL  HEAT MAP OF DIE IN COIL
    induction_coil.png heat_map_die.jpg
  3. When the mold is placed inside the magnetic field, the magnetic field induces current in the section exposed to the coil (See Figure 19). The mold is made of magnetic alloy Stainless Steel or Nickel (with high permeability) that can couple with the magnetic field, the mold material having internal resistance will cause heat buildup as the current, generated by the magnetic field, passes through the die surface. Higher the die material permeability the less energy will be required for heating the mold. CAUTION: DO NOT overheat the die. If the die is overheated it will lose its magnetic properties and will not heat properly. (The magnetic properties of 400 Series Stainless steel material increases 9 times after heat treating). If the heat treatment is not properly controlled from die to die it can cause process variations.
  4. Critical factors for heating the mold:
    • The RF power (current passing through the coil), higher power results in stronger magnetic field.
    • The duration of time the RF is energized
    • The distance between the die and the coil ID.
  5. Critical factors for melting the plastic
    1. The Insert Delay: which allows a set time to rise the mold temperature before the catheter is inserted into the mold.
    2. The Insertion speed: The speed that catheter is inserted into the mold, it must be adjusted to match the melting rate, which is controlled by a flow control valve.
    3. The Insertion Force: A pressure regulator is used to control the insertion force into the Mold.

What is Induction Heating?

Induction heating is a process which is used to heat a magnetic material inside a electromagnetic coil.

The basic principles of induction heating have been understood and applied in the industrial applications since the 1920s.

During World War II, the technology developed rapidly to meet urgent wartime requirements. More recently, the focus on lean manufacturing techniques and emphasis on improved quality control have led to a rediscovery of induction technology, along with the development of precisely controlled, all solid state induction power supplies.

In the most common heating methods, a torch or open flame is directly applied to the metal part. But with induction heating, heat is actually "induced" within the part itself by circulating electrical currents.

Induction heating relies on the unique characteristics of radio frequency (RF) energy.

How Induction Heating Works

The basic induction heating system can use a 50KHz to 13.56MHz frequency RF generator. The RF output signal is sent through a coil rapped around the die. As per Faraday’s low The coil serves as the transformer primary and the part to be heated becomes a short circuit secondary. When RF is going through the coil it creates magnetic filed and when a metal opject is placed inside the coil magnetic field, circulating eddy currents are induced on the surface of the part.

Induction_SetUp.png

 As shown in the second diagram, the eddy currents flow against the electrical resistivity of the metal mold, generating localized heat. This heating is often referred to as the Joule's first law.

 Secondarily, additional heat is produced within magnetic parts through hysteresis – internal friction that is created when magnetic parts pass through the coil. Magnetic materials naturally offer electrical resistance to the rapidly changing magnetic fields within the inductor. This resistance produces internal friction which in turn produces heat.

IMPORTANT FACTORS TO CONSIDER
The efficiency of an induction heating depends on several factors: the characteristics of the die outer shape, the design of the induction coil or, distance from the coil and generator power and frequency.

joules_image.png

 

MAGNETIC OR NON-MAGNETIC

It is easier to heat magnetic materials. In addition to the heat induced by eddy currents, magnetic materials also produce heat through what is called the hysteresis effect (described above). This effect ceases to occur at temperatures above the "Curie" point - the temperature at which a magnetic material loses its magnetic properties. The relative resistance of magnetic materials is rated on a “permeability” scale of 100 to 500; while non-magnetic materials have a permeability of 1, magnetic materials can have a permeability as high as 500.

THICK OR THIN

With conductive materials, about 85% of the heating effect occurs on the surface or "skin" of the part; the heating intensity diminishes as the distance increases from the surface. So small or thin parts generally heat more quickly than large thick parts.

The relationship between the frequency of the alternating current and the heating depth of penetration: the higher the frequency, the shallower the heating in the part. Frequencies of 100 to 400 kHz produce relatively high-energy heat, ideal for quickly heating small parts or the surface/skin of larger parts. For deep, penetrating heat, longer heating cycles at lower frequencies of 5 to 30 kHz have been shown to be most effective. Lower frequency applications required more power versus high frequency generators to heat the object.

RESISTIVITY

If you use the exact same induction process to heat two same size pieces of steel and copper, the results will be quite different. Why? Steel – along with carbon, tin and tungsten – has high electrical resistivity. Because these metals strongly resist the current flow, heat builds up quickly. Low resistivity metals such as copper, brass and aluminum take longer to heat. Resistivity increases with temperature, so a very hot piece of steel will be more receptive to induction heating than a cold piece.